A microwavehyphen;sideband CO2hyphen;laser optothermal spectrometer with a resolution better than 1 MHz has been used to record the infrared spectrum of Arndash;NH3in the vicinity of theaR(0,0) line of the ngr;2vibration of free NH3. A Pgr;larr;sum; type band is observed, giving a positivelhyphen;type doubling constantq, of 90.9 MHz for the upper state. The positiveqindicates that thej=1,k=0, sum; state is above thej=1,k=0, Pgr; state in thev2=1 excited state, wherejandkspecify the correlation of the internalhyphen;rotor state of the Arndash;NH3complex to the NH3monomer rotational statej,k. The ngr;2vibrationally excited complex is found to predissociate in less than the 0.9 ms transit time between the bolometer detector and laserhyphen;excitation region. A lower limit to the upperhyphen;state lifetime can be obtained from the observed linewidths, which range from 1.5 to 3 MHz (FWHM). The present results agree with and extend the previous freehyphen;jet diodehyphen;laser absorption measurements on this band.
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